This application claims the priority of German Patent No. 197 386 22.9, filed Sep. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an exhaust line for an internal combustion engine exhaust system having a catalytic converter.
An exhaust manifold is known from DE 40 39 735 A1 whose wall is made of three layers in cross section. The advantage of this exhaust manifold consists in the fact that the response time of a catalytic converter located downstream is shortened and total pollutant emissions reduced.
The exhaust manifold has an externally located metal pipe made especially from a stainless steel. Internally the wall has a damping layer connected forcewise with the pipe. The damping layer is composed of a foil and an insulating layer that coats and heat-insulates the foil. The foil is in direct contact with exhaust, while the insulating layer forms an intermediate layer between the external pipe and the internal foil of the exhaust manifold. The foil is made of a material that is resistant to heat and scale and has a layer thickness of less than 0.5 mm and especially 0.2 mm. The insulating layer consists of a high-strength and asbestos-free heat-damping material, for example a ceramic fiber made of SiO.sub.2, and has a layer thickness between 2 mm and 4 mm. The layer thickness of the damping layer composed of the insulating layer and the foil is therefore between 2.2 mm and 4.5 mm. The forcewise connection between the damping layer and the tube takes place along individual spot and/or linear welds between the foil and the pipe so that the insulating layer in the ideal case is clamped between the foil, the pipe and the individual spot and/or linear connections.
To line the pipe, the damping layer is initially shaped into a tubular, flexible, hollow body and folded at least once in the lengthwise direction, with the mass of the hollow body corresponding to that which it later has in the exhaust manifold. The folded hollow body is introduced into the outer pipe and applied free of folds to the inside wall of the outer pipe by a mechanical and/or hydraulic widening process. The manufacture of the previously known exhaust manifold therefore involves several successive worksteps and is therefore time-consuming, complex, and expensive. Furthermore the inside cross section of the outer pipe is reduced by at least 4.4 mm, so that the outside diameter of the outer pipe must be correspondingly large, involving increased weight, increased material consumption, and a greater consumption of space. This also has a negative effect on manufacturing costs.
A collecting pipe for exhaust is known from DE-OS 24 48 482 with a plurality of stubs that lead to the cylinders of an internal combustion engine. The collecting pipe and stubs are provided internally with an insulating layer that consists of several prefabricated and appropriately shaped individual pieces. The individual pieces are introduced into the collecting pipe and into the stubs, with the individual pieces located in the stubs engaging suitably shaped recesses that engage the individual pieces of the collecting pipe and hold these individual parts of the collecting pipe. In this case also, the manufacture of this exhaust pipe that consists of the collecting pipe involves a considerable space requirement in addition to being costly and tedious.
An outlet duct located in the cylinder head is known from U.S. Pat. No. 4,526,824, the duct constituting the exhaust line and having a heat-insulating inserted bushing on portions of its inside wall. The inserted bushing has an insulating layer of fibers facing the cylinder head and also has a sintered body made of heat- and scale-resistant aluminum titanate facing the exhaust. To produce the outlet duct, that is the exhaust line, the prefabricated bushing is defined in terms of position before the cylinder head is cast in its mold and is largely inserted in a fixed position, and the cylinder head is then cast. Since the fixed mounting in the casting mold involves tolerances, some of which are considerable, the outlet duct must at least be checked after casting and finished as necessary. For these reasons among others, this type of manufacture is likewise tedious and expensive.
A muffler is known from EP 27 009 which is provided internally at least area wise with an enamel layer for noise damping. In addition, various methods for applying the enamel layer are known from EP 27 009 as well as various physical designs for the surface of the muffler for improved adhesion of the enamel layer.
Finally, there is air gap insulation in which the exhaust manifold is made from an inner pipe and an outer pipe located concentrically with respect to one another and spaced apart from one another in a heat-insulating fashion by an air gap. Air gap insulation is likewise very cumbersome and expensive.
An object of the present invention is to provide an improved exhaust line so that with a comparable reduction of total pollutant emissions, the manufacture of the exhaust line is simplified and made as inexpensive as possible.
This object has been achieved by an exhaust line with a heat-conducting layer and an insulating layer connected directly and forcewise with the heat-conducting layer, the insulating layer having a thickness of less than 2 mm, having a specific heat capacity of less than 10.sup.6 J/(m.sup.3 h) and having a thermal diffusivity of k of less than 0.01 cm .sup.2 /s. Surprisingly, despite the fact that the very thin insulating layer can be applied internally both simply and economically, the exhaust line is adequately heat-insulated. Since the layer thickness of the insulating layer is also thin, the flow cross section is advantageously not affected adversely.